This invention relates to an optical receiver and an optical receiving method for use in receiving a sequence of data signal. Herein, it has been noted throughout the instant specification that this invention may be applicable to any other receivers and receiving method than the optical receiver and optical receiving method, although the following description will be mainly restricted to the optical receiver and the optical receiving method.
Recent expectations have been directed to an optical transmission system that can transmit a large amount of data signals at a high speed. A wide variety of applications have been considered about the optical transmission system. In such an optical system, an optical amplifier and a wavelength division multiplex (WDM) technique have been often used to establish a high speed transmission.
However, the above-mentioned optical transmission system is disadvantageous in that a received wave-form is liable to be distorted due to various degradation factors increased in number in comparison with any other conventional optical transmission systems, although the former make it possible to transmit the data signals at a high speed as compared with the latter.
For example, the optical transmission system very often includes, as the optical amplifier, an optical fiber amplifier formed by an optical fiber. Such an optical transmission system has, as the degradation factors of the waveform, noise resulting from amplified spontaneous emission (ASE) generated by the optical fiber amplifier and dispersion of an optical fiber. In addition, the waveform distortion also takes place in the optical transmission system due to a nonlinear effect and a cross talk from an adjacent channel in the WDM. Especially, the waveform distortion resulting from the nonlinear effect becomes serious with an increase of optical signal power in the optical fiber.
Heretofore, an optical receiver included in the above-mentioned optical transmission system usually has a clock and data recovery circuit for recovering or regenerating the data signals at a predetermined threshold level. However, a CDR of this type can not always correctly discriminate or regenerate the data signals deteriorated in waveform due to the dispersion of the optical fiber, and the like.
Alternatively, a proposal has been also made about a clock and data recovery circuit that has a function of adjusting a threshold level of the data signals to an optimum level. This circuit can correctly discriminate the data signals deteriorated in waveform and will be called a clock and data recovery circuit (CDR) with a threshold level adjusting function.
However, the CDR with the threshold level adjusting function optimizes only the threshold level of the data signals but can not compensate for waveform distortion or degradation itself. This shows that the CDR mentioned above has a limit to correctly regenerate the data signals.